In the scheme of things, there are generally two types of powders; powders that exhibit good flowing characteristics and powders that can be best described as non-flowable types. It goes without saying that flowable powders are relatively easy to continuously transfer from one location to another. On the other hand, non-flowable powders are difficult to transfer.
In particular, we were faced with the problem of feeding a mixture of carbon and Teflon from a high speed intensifying mixer to a subsequent strip manufacturing step. (Teflon is a registered trademark of E. I. duPont deNemours and Company.)
The expression "flowable powder" is being used in a very broad sense. Powders do not really flow in a strict hydraulic sense but rather the particles constituting the powder slide and roll over each other; the particles being driven by an external force (gravity, a fluid, etc.). Accordingly, the "flow" characteristics of the powder, are dictated, in part, by the particles' self-adhesive properties. Obviously, particles having a relatively slippery or smooth (ex. ball bearings) external surface will exhibit better flowability characteristics than particles having a rough or sticky surface.
In our case, it is the Teflon constituent that causes the carbon/Teflon mixtre to exhibit extremely poor flowability characteristics. The mixture is a very light, fluffy and spongy material that tends to adhere to itself which creates clumps, thereby making a continuously flowing feeding process extremely difficult. Moreover, it is imperative that there be no agglomerates in the mixture, since the clumped material tends to precipitate undesirable holes in the manufactured sheet.
Accordingly, it was necessary to develop a feeding apparatus that could: (1) continuously feed a non-flowable powder to a subsequent manufacturing step and (2) simultaneously prevent particulate formation in the powder during the feeding process.